How did the land invasion of Europe happen so quickly post D-Day as opposed to today, where it takes 8 months to liberate one city?

I've been reading Eisenhower's book (Crusade in Europe) about the operations during WW2.

Several factors come to mind:

The number of soldiers involved in WW2 was staggering. Operation Overlord brough 2 million soldiers to bear by end of August 1944.
A number of biggest cities had population which welcomed the US (and Brittish, and French, and Canadian and Polish, and a mix of others) troops. Which made it hot ground for the Germans, who therefore retreated to more easily defended positions.
A number of the biggest cities on the western front were declared open cities: fighting took place around them, but not IN the cities.
The speed you note was on the western front, but not on the Eastern front. Compare the sieges of Leningrad and Stalingrad, and the taking of Berlin. In those, fighting was street-to-street, and the local population actively resisted.
In the battle of Berlin, it took 2.5 million soldiers (with 6000 tanks, 7,500 aircraft and 41,000 artillery pieces) a month to take a city defended by a remanent of a once mighty force short of weapons, ammunition or supply, without sparing any regard to preserving civilians or infrastructure.
The siege of Sarajevo lasted 1425 days, and arguably what saved Sarajevo was not the force of the arms within, but the attention of the media from without. If the Serbian army had tried to do to Sarajevo what the Red Army did to Berlin, other countries might have been moved to action.

I hope I've given you enough pointers, witout once mentioning anything after 1997.

I'd like to add that the speed of modern warfare, like gen. Schwartzkopf's Hail Mary pass (and the attendant, very important, logistic feats) would have filled Montgomery, Patton and Eisenhower with admiration.